Spaceborne systems for making scientific observations of the earth tend to be large and specialized for a particular type of measurement, such as ocean altimetry or atmospheric chemistry. Platforms that attempt to provide diverse types of observations simply carry a variety of costly, specialized instruments and can be as large as a city bus. The average cost of a focused Earth science mission at NASA is approaching $1B. The cost of the larger, multi-purpose observing platforms, like NASA's EOS spacecraft (Terra, Aqua, and Aura) and NOAA's NPOESS and GOES-R, has surpassed $3B. These costs have discouraged the use of large constellations in low orbit that can observe virtually the entire earth at close hand all at once. That situation is about to change. The same micro-electronic and wireless technologies that give us vest-pocket super-phones for $300 can be brought to bear on spaceborne observing systems and reduce their size and cost by up to two orders of magnitude. This opens the door to an entirely new kind of Earth observing system comprising many tiny, free-flying “cells” that are individually simple but that together can perform the sensing functions of a dozen or more of today's bulky, highly specialized observing platforms, and at far lower cost.
The nature of the prior art is evident from (a) the Earth observing missions now flying; (b) the list of Earth missions planned or proposed by NASA and NOAA over the next 15 years; (c) patents for new remote sensing mission and measurement concepts; and (d) mission and measurement concepts described in the open literature.
(a) Current Earth Observing Missions
FIG. 1 shows an essentially complete collection of current Earth observing missions operated by NASA and NOAA. NASA missions constitute the outside ring and NOAA missions the inner satellites. Counting duplicate NOAA spacecraft not shown, there are about 30 currently operating missions carrying more than 120 individual instruments. These missions constitute two classes: (1) the “super-platforms” carrying a variety of different instruments (Terra, Aqua, Aura, GOES, NOAA-N), which are now in the $3B cost class, and (2) what we will call “focused” missions—lower-cost missions designed to address a narrower set of scientific questions. The latter generally cost in the 300-800 M$ range in today's dollars. Whereas the super-platforms may be the size of a city bus, the focused mission spacecraft tend to be about the size of a car or minivan.
Each of these missions was designed to perform specific functions. GRACE is a twin-satellite system to measure the earth's gravitational field; IceSAT performs laser altimetry over ice and water; Jason-1 and OSTM perform radar altimetry over the oceans; CloudSat and CALIPSO measure cloud properties; TRMM observes tropical rainfall; QuikSCAT performs ocean scatterometry; Landsat focuses on land surface properties, and so on. Even the super-platforms have a targeted, if somewhat broader focus: Terra on land observation, Aqua on water, Aura on atmosphere, POES (NOAA-N) on weather. To the extent that there is an Earth-observing constellation it is simply an ad hoc collection of large, costly platforms, each tailored for unique scientific and observational objectives.
Emerging Constellations:
In some commercial and governmental quarters the idea of rudimentary remote sensing constellations is taking hold, primarily for Earth imaging. (There are also large commercial constellations for global telecommunications, which is outside our domain of interest.) Germany is putting up their TerraSAR-X and Tandem-X radar imaging system [1]; Italy is planning a five-satellite COSMO-SkyMed radar imaging system [2]; DigitalGlobe, GeoEye, and RapidEye are planning few-satellite optical imaging systems [3]-[5]. These, however, are simply multiple copies, in small numbers, of larger, dedicated platforms designed for a single purpose. All are devoted to Earth imaging in one form or another. A slight departure from this is the COSMIC system of six GPS radio occultation satellites funded primarily by Taiwan [6]. While these too are focused on a single purpose, the individual satellites are smaller and lower cost than the others thus far mentioned. CICERO will carry this miniaturization much further and introduce incomparably greater functional versatility.
(b) Proposed Earth Observing Missions
FIG. 2 lists the 15 top-priority new missions proposed for NASA for the next 10-15 years. This list is the result of a major “Decadal Survey” conducted by the National Research Council for NASA and NOAA and released in January 2007 [7]. Also shown are the nominal mission costs in 2007 dollars as estimated during the Decadal Survey and independently by NASA. Apparent from this list (and from the detailed reports and mission design studies) is that there is no hint of movement away from the current paradigm of large, costly, single-purpose, single-platform missions, each individually designed for narrow observational objectives. Indeed, this list moves even further down that road. Each proposed mission requires a substantial base platform. The average mission cost as estimated by NASA (which provides the best-informed estimates) is $711 M in 2007 dollars, or nearly $1B each at the time the missions are proposed to fly. A similar, somewhat shorter list from the European Space Agency (ESA) shows exactly the same pattern [8],[9]. There is no thought yet by the major agencies of breaking from this model of costly, focused observing platforms, though there is some hint that there will be fewer of the super-platforms in the future.
The Decadal Survey also proposed two new operational NOAA missions not shown in FIG. 2: A mission to measure total solar irradiance and a COSMIC follow—on mission to perform Global Navigation Satellite System radio occultation (GNSS RO). The COSMIC follow-on, however, is presented strictly as an update to COSMIC: a constellation of 6-12 satellites focused exclusively on radio occultation of the atmosphere. There is no hint of larger numbers or greater functional versatility.
(c) Patents for Mission and Measurement Concepts
A patent search for similar or related ideas turned up nothing resembling our cellular, multi-function CICERO concept. All ideas for Earth observing systems or measurements tend to focus on a particular type of measurement for a particular observational purpose, very much in the paradigm of the current and proposed mission lists. The search did, however, turn up various ideas relevant to CICERO in one way or another. We list here the most pertinent of these with some brief comments.
U.S. Pat. No. 4,727,373—Method and system for orbiting stereo imaging radar, Feb. 23, 1988: This describes a tethered system for stereo (and presumably interferometric, though that is not mentioned) SAR imaging. CICERO will be able to accomplish the same thing with non-tethered, free-flying cells through use of centimeter-level, GNSS-based precise orbit determination to precisely co-register independently acquired images.
U.S. Pat. No. 4,990,925—Interferometric radiometer, Feb. 5, 1991: This describes a particular technique involving an interferometer on a single platform to map the radio or microwave intensity pattern of a given scene with high resolution. Again, CICERO will be able to perform a similar operation by precisely co-registering radio or microwave data acquired independently by two or more cells.
U.S. Pat. No. 5,546,087—Altimetry method, Aug. 13, 1996: This patent by a French group proposes the now well-known method of bistatic radar altimetry with radio signals of opportunity, particularly those from global navigation satellites. While this was submitted in October 1994, there are documented proposals for precisely the same technique by US groups at least as early as 1991, though they did not seek patents. CICERO will be able to perform this method of altimetry as one of its many possible observational functions.
U.S. Pat. No. 5,552,787—Measurement of topography using polarimetric synthetic aperture radar, Sep. 3, 1996: This proposes a “polarimetric” SAR technique for measuring “terrain azimuthal slopes and a derived estimate of terrain elevation.” CICERO will be able to provide similar information with more conventional SAR and interferometric SAR (InSAR) techniques. Since CICERO also preserves precise signal polarization information, it could in principle allow use of this technique as well, though just how well remains to be investigated.
U.S. Pat. No. 5,608,404—Imaging synthetic aperture radar, Mar. 4, 1997: This proposes a particular technique for efficiently forming a SAR image by first collecting the returned signals in a set of subaperture antenna elements, initially processing each subaperture array separately to obtain coarse-resolution in azimuth, then merging subaperture results to obtain full aperture resolution. This is essentially an efficient SAR processing technique for the acquired data. CICERO will collect radar data with an array of many sub-elements and thus this technique could in principle be applied in the processing, although other techniques are available as well. Methods of processing SAR data are outside the scope of interest of the present invention.
U.S. Pat. No. 5,931,417—Non-geostationary orbit satellite constellation for continuous coverage of northern latitudes, Aug. 3, 1999: This proposes a particular arrangement of elliptical orbits for Earth observing spacecraft that would provide continuous coverage of northern latitudes from below geostationary altitude. CICERO instead will use larger numbers of cells in low circular orbits. With enough cells, CICERO will provide continuous coverage of the entire globe from a very low altitude.
U.S. Pat. No. 5,936,588—Reconfigurable multiple beam satellite phased array antenna, Aug. 10, 1999: This proposes a method of controlling a phased array antenna to form two or more simultaneous beams and steering them in real time to desired target points. The method has the disadvantage that different antenna elements are used to form the different beams and thus the gain of each simultaneous beam is less than the gain of the full antenna array. (The sum of the beam gains equals the total antenna gain.) CICERO will not steer the beam of its phased array antenna to particular points in real time. Rather, it will preserve the signal information arriving at each array element so that the full gain of the array can be steered arbitrarily to any number of points simultaneously long after the data have been acquired.
U.S. Pat. No. 6,011,505—Terrain elevation; measurement by interferometric synthetic aperture radar, Jan. 4, 2000: This proposes a method of processing radar data to form corrected SAR images and of combining image pairs to form a SAR interferogram yielding accurate terrain elevation information. It is a processing technique. It is not clear to us how this technique differs from previously demonstrated InSAR techniques that yield similar information. In any case, the technique (and others) could be readily applied to CICERO data to form SAR images and interferograms. The essence of CICERO is that it provides raw data that can be combined and processed in myriad ways for many purposes besides SAR and InSAR; this technique can surely be used as well.
U.S. Pat. No. 6,130,644—Method and apparatus for geodetic surveying and/or earth imaging by satellite signal processing, Oct. 10, 2000: This proposes a method of forming SAR interferograms using reflected signals observed both by the satellite(s) and by at least one directional antenna fixed relative to the ground. While there are other satisfactory ways of forming the interferograms, this can offer an enhancement and is perfectly suitable for use with CICERO data for anyone wishing to install such ground antennas, which are not inherent in the CICERO system.
U.S. Pat. No. 6,264,143—Radar interferometry device, Jul. 24, 2001: This proposes a configuration of satellites for obtaining InSAR measurements of the earth's surface. The configuration involves at least one emitter satellite and a constellation of receiver satellites. So far, this is similar to the radar function (and only the radar function) of CICERO. However, the receivers are specified to be “ . . . accurately synchronous and their orbits [to] have the same eccentricity which is different from that of the orbit of the emitter. During one orbital period, the satellites travel round a relative ellipse over which they are uniformly distributed. The invention [applies] specifically to measuring ocean currents, measuring world topography, and differential interferometry.” This is a very particular configuration for a particular type of InSAR measurement. CICERO does not reproduce or mimic this configuration. CICERO will, however, be able to provide equivalent observational information, again by exploiting centimeter-accuracy GNSS-based POD for all cells to precisely co-register data from multiple cells.
U.S. Pat. No. 6,388,606—Aircraft or spacecraft based synthetic aperture radar, May 14, 2002: This proposes a type of bistatic SAR system (i.e., a system wherein the emitting and receiving elements are separate) in which both the emitter and receiver can be moving, as they are with CICERO. The uniqueness of this invention lies in its use of different beamwidths for the transmit (wide beam) and receive (narrow beam) antennas, and other methods to suppress antenna sidelobes and facilitate ambiguity removal in determining the point of reflection. This technique can indeed be mimicked with CICERO by proper combination of the data from the multiple distinct antenna elements on each cell. The uniqueness of CICERO lies not in any novel method of SAR imaging, but in its novel architecture that allows many types of radar sensing (not just imaging) as well as many other forms of non-radar Earth, atmospheric, and ionospheric sensing.
U.S. Pat. No. 6,400,306—Multi-channel moving target radar detection and imaging apparatus and method, Jun. 4, 2002: This proposes a type of radar imaging involving an illuminator and multiple receiving apertures on aircraft or in space. The uniqueness of the invention lies in its use of “space-time adaptive processing (STAP) algorithms to better compensate for channel mismatches, better suppress stationary clutter, and to suppress main beam jamming,” leading to a claimed improvement in moving target detection. Again, this sort of tailored radar processing is outside of the area of claims of the present invention. We note, however, that because CICERO will preserve the essential information in the signals received at each antenna element, it will permit such tailored techniques to be applied in processing and thus their benefits to be realized.
U.S. Pat. No. 6,452,532—Apparatus and method for microwave interferometry radiating incrementally accumulating holography, Sep. 17, 2002: This proposes a particular arrangement of orbits for space-based bistatic SAR imaging, wherein the receiving satellites cannot generally observe the same target spot at the same time. (This necessarily closes off the possibility of imaging moving targets.) In this proposed configuration, satellites are placed in three precisely prescribed, mutually orthogonal orbit planes, the criticality of which is insisted upon by the inventor (though, to this reader, the reasoning is obscure). CICERO has no such constraint and does not employ such orbits. CICERO can image moving targets at high resolution with multiple receivers viewing the same target at once, can image stationary targets with data acquired at different times, and can perform all other functions claimed for this invention without its rather substantial limitations.
U.S. Pat. No. 6,586,741—Method and system for two-dimensional interferometric radiometry, Jul. 1, 2003: This proposes an interferometric technique for imaging a planetary surface with received thermal (i.e., infrared) radiation. As the baseline CICERO system will not include thermal radiation sensors, this invention is only minimally relevant.
U.S. Pat. No. 6,844,844—System comprising a satellite with radiofrequency antenna, Jan. 18, 2005: This proposes a design for a space-based phased-array radar antenna. The unique aspect of this invention is a means of controlling the antenna beam “so as to keep the orientation of a beam . . . unchanged in the reference frame associated with the antenna in spite of modifications to the orientation of the illumination direction used by the beam . . . ” CICERO will not use this method of control but instead will broadcast a constant radar beam downward over a wide angle. As noted previously, the receiving antenna beam shape can be arbitrarily modified after the fact by appropriately combining the data preserved from each element of the phased array antennas.
U.S. Pat. No. 6,870,500—Side looking SAR system, Mar. 22, 2005: This proposes a SAR antenna configuration not unlike that of U.S. Pat. No. 5,608,404 in which the observing aperture is “divided into a number of . . . sub-apertures arranged in the elevation and azimuth directions.” Special real-time circuitry is used to phase shift each receive sub-aperture signal and sum them so as to maximize the resultant received signal amplitude. CICERO will not attempt any tailored, real-time phase shifting and combining of signals received at each antenna element. Once again, because CICERO will preserve the raw signal information received at each element of a phased array antenna, such phase shifting and signal combining can be performed in myriad ways on multiple received signals, without prior knowledge of their direction of origin, long after the data have been acquired.
U.S. Pat. No. 6,911,931—Using dynamic interferometric synthetic aperture radar (InSAR) to image fast-moving surface waves, Jun. 28, 2005: This proposes a particular differential method of processing InSAR data to image fast moving surface waves. The method uses radar data acquired from multiple moving platforms, generated by at least one transmitter. No particular constraints are required of the transmitter or receiver. Thus the method can be applied to CICERO radar data. This illustrates yet another special application to which the CICERO architecture lends itself.
U.S. Pat. No. 7,196,653—Imaging apparatus and method, Mar. 27, 2007: This proposes a SAR imaging technique in which multiple transmit beams illuminate a scene and the returns are processed with the use of independent ground elevation data to determine the receiver attitude in all three axes. This again is essentially a processing technique, involving the introduction of external information (viz., a priori ground elevation data), which appears to be applicable to CICERO data as well, though it should not be needed since each cell will precisely determine and report its own attitude.
U.S. Pat. No. 7,348,917—Synthetic multi-aperture radar technology, Mar. 25, 2008: This proposes techniques for reducing the antenna size or increasing the swath width without increasing ambiguities in SAR imaging systems. It involves transmitting radar pulses at regular intervals having only a portion of the intended SAR bandwidth and then extrapolating the received signals to the full bandwidth. This rather specialized technique could in theory be implemented on CICERO, though at present there is no plan to do so.
U.S. Pat. No. 7,414,573—Method and apparatus for providing an integrated communications, navigation and surveillance satellite system, Aug. 19, 2008: This is something of a departure from the others in that it proposes not a remote sensing system but an integrated telecommunications and user positioning system, which can also report the user position to others. It comprises a constellation of satellites that broadcast positioning signals (like GPS) and provide two-way user communications (like Iridium, OrbComm, and Globalstar). It does not offer any capability for Earth remote sensing from space. It is nevertheless of interest here because CICERO, with its integrated transmit and receive functions for Earth observation and its need to communicate its gathered information between cells and to ground sites, will also, without modification, be able to provide ground and near-earth user positioning, messaging, and surveillance, though its principal function is Earth remote sensing. This points up the power and versatility that is achieved with the cellular design. The simple transmit, receive, and relay functions offer great breadth of possible uses that today are achieved, if at all, with distinct and uniquely tailored system architectures. Many new uses will emerge that have not yet been thought of, as has happened with GPS itself.
Consider again the Earth missions of FIGS. 1 and 2: Once one has gone to the trouble of tailoring a spacecraft design for a particular set of specialized functions, one finds that: (a) the spacecraft becomes larger, more complex, and more costly; and (b) it can generally perform little more than those functions for which it was tailored. The more primitive operations of CICERO cells leave open a host of potential applications, much like the simple AND, OR, and NOT operations of a basic digital logic circuit can make possible almost any conceivable computation.
(d) Mission Concepts in the Open Literature
An extensive search of the relevant literature for novel mission concepts turned up essentially nothing beyond what we have described above. Most published ideas deal with variations on mission concepts already flying or proposed. Indeed, the most comprehensive attempt to gather new mission concepts was performed by the NRC Decadal Survey panel, which conducted an open solicitation (RFI) of new Earth mission concepts for the next decade and received scores of replies. The best of these are reflected in the recommended list shown in FIG. 2. If there is a mission concept out there resembling CICERO in its simplicity and generality, we have not discovered it.